Visible-range NPLs (A = 585.3, 703.2, and 724.5 nm) on 3p-3s transitions of the Ne atom were studied at VNIIEF, VNIITF, and the Sandia Laboratories (see Chap. 3, Sect. 3.2). In contrast to NPLs operating on the nd-(n + 1)p transitions of Xe, Kr, and Ar atoms, in neon NPLs the main processes leading to the formation of an inverse population of levels were established rather reliably [4, 83—92]. Already in the first studies of neon lasers excited by nuclear radiation and electron beams there are nearly identical notions of the lasing mechanism, which may be explained by the extensive spectroscopic information about the processes of populating and relaxation of 3p and 3s states of the Ne atom (see [10], for example). In particular, populating of the level 3p'[1/2]0 owing to dissociative recombination of molecular ions Ne+ with electrons was proven earlier in study [93], where additional Doppler broadening of the 583.3-nm line in the afterglow phase was observed, and the suggestion of using the Penning reaction to depopulate the lower laser states was demonstrated as early as 1970 (for example, see [1]). Because the lower 3s states of the Ne atom are depopulated in the Penning reactions

Ne * (3s) + M — M+ + e + Ne,

sometimes such lasers are called Penning lasers. When nuclear pumping was used, the admixtures M = Ar, Kr, Xe, and H2 were used to depopulate the 3s levels.

During the development of theoretical models, fundamental attention was paid to the transition 3p'[1/2]0-3s'[1/2]10 of the Ne atom (A = 585.3 nm), where lasing was most efficient when using the ternary mixture He-Ne-M (a diagram of Ne atom levels with lasing transitions is shown in Fig. 3.3). Formation of an inverse population in the He-Ne-M mixture occurs as a result of the following sequence of basic plasma processes:

In this sequence there are three stages at which process competition takes place that influences the population of the upper lasing level:

(a) Competition of charge-transfer processes

He) + Ne — Ne++ 2He, (5.10)

He) + M — M++ 2He; (5.11)

(b) Competition of dissociative recombination

Ne+ + e — Ne * (3p) + Ne (5.12)

and the charge-transfer process

Ne+ + M — M++ 2Ne; (5.13)

(c) Competition of radiative transition at the 585.3-nm line and the quenching processes of level 3p'[1/2]0 owing to the Penning reaction, as well as in collisions with He and Ne atoms. Therefore the populating efficiency of level 3p'[1/2]0 will depend on the type of quenching admixtures M, the pressure and composition of the mixture, and the electron concentration (specific power deposition).

If a rare gas (M = Ar, Kr, or Xe) is used as the quenching admixture, then in the second phase the losses are not significant, because the processes (5.13) for these atoms have low rate constants (< 10~13 cm3/s [94]). Replacement of Ar(Kr, Xe) by H2 leads to a marked competition of the processes in the second phase, because the rate constant of the process (5.13) for H2 molecules is quite high—4.2 x 10~n cm3/ s [95]. Therefore in NPLs based on 3p-3s transitions of the Ne atom, it is expedient to use hydrogen as the quenching admixture only at high specific power depositions
(q > 1 kW/cm3) [96]. We note that in the model [90, 91], the rate constant of process (5.13) with participation of the H2 molecule, determined during the process of numerical modeling, proved to be three orders of magnitude less (1.3 x 10-13 cm3/s). This low value contradicts the above data of [95] and the results of measurement [84] of the luminescence intensity at X = 585.3 nm in the He-Ne-Ar mixture as a function of the concentration of Н2. As the data in [84] show, for q « 1 kW/cm3 a twofold reduction in intensity (reduction in population of the upper lasing level by a factor of 2) occurs at H2 pressure of about 1.5 Torr, while the influence of H2 is determined precisely by process (5.13). The rate constant of this process, estimated from the conditions of the experiment [84], is no less than 5 x 10~n cm3/s, which agrees with the data of study [95].

In studies [89, 90], the models included the ternary charge-transfer reaction, which for M = Ar has the view:

Ne+ + Ar + R! Ar+ + 2Ne + R, (5.14)

where R is the third particle (in this case, He, Ne, or Ar). The rate constant of this reaction in model [90] for R = Ne was taken as equal to 5 x 10~32 cm6/s. The model [89] used a significantly higher value of this constant (3.5 x 10~30 cm6/s), which agrees with the data of [97]. The question of the need to include process (5.14) in the models remains open for now because the results of luminescence investiga­tions [84] testify to an insignificant influence of this process on the concentration of Ne2+ ions.

To calculate lasing characteristics, it is necessary to determine the populations of the 3p levels of the Ne atom. To this end, usually a system of kinetic equations is solved which constitutes a balance of the rates of populating and relaxation processes for each of ten 3p levels. Of the basic processes, it is necessary to allow for populating of these levels through reaction (5.12) and the collisional intra-multiplet transitions between them, the radiative and collisional relaxation of the 3p levels during collisions with the He and Ne atoms, as well as the Penning reaction for each of these levels with the participation of M atoms.

Reaction (5.12) is the main populating process of 3p levels, and in most of the studies it was assumed that all of these levels are populated during recombination of molecular ions Ne+ in the ground vibrational state. A somewhat different mecha­nism was considered in studies [87, 88], where it was assumed that level 3p'[1/2]0 (upper among the 3p levels) is populated with the participation of vibrationally excited ions Ne+. The percentage of dissociative recombination flux of (5.12) reaching level 3p'[1/2]0, according to various information, is around 7 % at a low neon pressure [10], 3-17 % when neon pressure is reduced from 760 to 10 Torr [87, 88], and 7-10 % for 20-100 Torr [98]. In ternary mixtures He-Ne-M with a large content of helium, the efficiency of populating level 3p0[1/2]0 is increased [87, 88], which the authors explain by the formation of vibrationally excited ions Ne2+ in the processes:

Ne + 2He! HeNe+ + He,
HeNe+ + Ne! Ne+ (o > 0) + He.

The relationship between the processes of dissociative recombination of rare gas molecular ions with the kinetics of populating of their vibrational levels was studied in the afterglow of a pulsed gas discharge (see [10], for example).

Lasing was also observed in neon NPLs at the 703.2- and 724.5-nm lines, which start from level 3p[1/2]1. This level is effectively populated both through process (5.12), and as a result of intramultiplet collisional transitions from the higher 3p — states.

One more mechanism of populating of 3p levels, proposed in study [53] and based on the investigation of the kinetics of stopping fission fragments in neon and a He-Ne mixture, consists of direct excitation of these levels by nuclear particles. However, as was demonstrated by experimental data [89], for He-Ne-Ar and Ne-Ar mixtures excited by an electron beam with a short duration of 3 ns, pulses of luminescent and laser radiation appear with a delay of up to 200 ns with respect to the pumping pulse, while its value is inversely proportional to the helium pressure. This delay can arise only in the case when laser states are populated not as a result of direct excitation, but owing to subsequent plasma processes. The authors [89] explain the onset of delay to electron “cooling” processes to temper­atures at which the rate of the recombination process (5.12) becomes significant and sufficient to achieve the laser threshold.

The population of 3p levels of the Ne atom is markedly influenced by collisional intra — and intermultiplet transitions during collisions with Ne [53, 99, 100] and He [101, 102] atoms. It is noted in studies [99, 100] that inter-multiplet transitions predominate for the 3p[1/2]1, which is the lowest among all the 3p states. At high specific power depositions, the processes of collisional relaxation of 3p levels during collisions with plasma electrons are added to these processes. For neon NPLs, the latter are less significant than, for example, for NPLs based on transitions 5d-6p of the Xe atom. In study [89] the conclusion was drawn that the effect of collisional mixing of the levels by electrons in a neon laser is not significant up to electron concentrations of 6 x 1015 cm~3.

Populating of the lower lasing level 3 s'[1/2]10 occurs through radiative and collisional intermultiplet transitions from 3p states. Depopulating of level 3 s’ [1/2]10 and three other 3s levels is carried out mainly through the Penning reaction (5.9), as well as through the reactions of associative ionization, intra-multiplet relaxation, and three-body processes with the formation of excimer molecules Ne2*. The processes of depopulating of 3s levels were considered most fully in studies [103, 104]. Table 5.9 shows the rate constants of Penning processes for all four 3s levels. We note for comparison that the rate constants of Penning processes for the atoms Ne*(3p'[1/2]0) with the participation of Ar and H2 are 5.3 x 10~n and 4.6 x 10~n cm3/s [105], respectively.

The most detailed kinetic models for NPLs based on 3p-3s transitions of the Ne atom [88—90] include up to 450 plasmochemical reactions [90]. For example, model [90] considered the atoms, molecules, and ions He*, Ne*, Ar*, Ar**, Ne2*, Ne2**, He2*, HeNe*, Ar2*, He+, Ne+, Ar+, He+, Ne+, Ar+, He+, Ne+, Ar^, and HeNe+, as well as the kinetics of population and relaxation of individual levels belonging to the groups of states Ne*(3 s,3 s’), Ne*(3p,3p’), Ne*(4 s), and Ne (5 s). The calculations were primarily carried out for the transition 3p'[1/2]0- 3 s’ [1/2]10 of the Ne atom (A = 585.3 nm). The kinetics of the neon NPL at the 703.2- and 724.5-nm lines (Ne-Kr mixture) were considered, evidently, only in studies [87, 88]. Experimental data obtained in pumping of a neon laser with nuclear radiation and electron beams were used for testing of the models.

The main differences of the models lie not in the quantity of plasma processes included in them, but in the use or absence of certain ones (for example, reaction (5.14)), differences in the rate constants for a number of important processes, and the probabilities of population of 3p levels through the reaction of dissociative recombination (5.12). One of the basic results of numerical modeling was the conclusion that the maximal efficiency of neon NPLs is no more than 0.5 % [90, 91].